One of the major advantages of fiber-optic gyroscope technology is that it offers the potential of a low-cost, all-solid-state approach with long-lifetime and high-reliability advantages over current gyroscopes. In order for these advantages to materialize, it is imperative that the fiber-optic gyroscopes have scale factor correction of 100 ppm or better, although exact requirements on scale factor are dependent on the application desired. Scale factor correction to this accuracy implies that elements which change the scale factor relationship must be monitored to a corresponding accuracy. One such element to be monitored is the output wavelength of the light source of the fiber optic gyroscope to an accuracy compatible with application requirements. Since the output of the light source of fiber optic gyroscopes depends on temperature and light source drive current in an unpredictable manner, a means to compensate or eliminate this scale factor error source is required.
For an open loop fiber optic gyroscope, the relationship between the detected output of the gyroscope and the rotation rate .OMEGA. is shown as follows. The detected output when phase sensitively demodulated, as is known in the art, is shown by ##EQU1## where K is approximately proportional to I.sub.o, the input intensity,
.lambda. is the wavelength of the light source, c is the speed of light in a vacuum, .phi..sub.s is the Sagnac phase shift between the counterpropagating waves, L is the length of the fiber, D is the coil diameter, and .OMEGA. is the angular rotation rate. When the rotation rate is small or is linearized, such as by an inverse sine function, the relationship is represented by ##EQU2##
The scale factor (SF) for the open loop gyroscope is rewritten as ##EQU3##
In a closed loop configuration, the output is utilized as an error signal for feedback, possibly using serrodyne concepts as are known to those skilled in the art. The relationship between the output of such a closed loop fiber optic gyroscope and rotation rate can be expressed in the form ##EQU4## where n is the index of refraction of a fiber coil of the fiber optic gyroscope, .lambda. is the wavelength of a light source of the fiber optic gyroscope, D is the diameter of the fiber coil, and .DELTA.F is the frequency difference between counterpropagating waves of the fiber optic gyroscope. The scale factor for determining the rotation rate .OMEGA. can therefore be rewritten as ##EQU5##
As shown by the above equations, in both the open loop and closed loop configurations, wavelength .lambda. needs to be controlled or compensated to compensate or correct scale factor. Since the wavelength of the light source varies with temperature, current, and other parameters, a change in scale factor is difficult to monitor and thus such a change is difficult to compensate or eliminate as is required for scale factor stability.
Such a change in scale factor can be approximated in accordance with the equation ##EQU6## for a closed loop configuration, and ##EQU7## for a open loop configuration, where SF is the scale factor, .lambda. is the wavelength, T.sub.S is the source temperature, I.sub.f is the source drive current, n is the coil's index of refraction, T.sub.c is the fiber coil temperature, L is the fiber length, and D is the effective diameter of the fiber coil. As is seen by the above equations, the temperature dependence of the scale factor is given by the temperature dependence of L, D, n and .lambda., with the change in wavelength .lambda. being the dominant term.
As suggested in the article, "Scale Factor Correction in the Phase-Nulling Optical Gyro", by E. Udd R. J. Michal and R. F. Cahill, Fiber Optic and Laser Sensors II, Proc. SPIE, Volume 478, pp. 136-141 (1982), the change in wavelength of the light source which varies with temperature, current and other parameters is monitored by tracking a separation in frequency between adjacent fringes of the gyroscope which depends on the wavelength of the systems light source. As the wavelength is then determined therefrom, the change in wavelength can be utilized to correct scale factor.
The article, "Fiber Optic Rotation Sensor: Analysis of Effects Limiting Sensitivity and Accuracy," by G. Schiffner, B. Nottbeck, and G. Schoner, Fiber-Optic Rotation Sensors and Related Technologies, pp. 266-274 (1982), discusses the stability of scale factor and the effects causing scale factor changes. One such effect is the change of laser wavelength from the source. It is indicated that the wavelength must be sensed and a correction of scale factor must be made for wavelength. The article indicates that such laser wavelength changes must be taken into account in the data processing unit for the fiber optic gyroscope and that if the wavelength is a function of temperature only, a temperature sensor would be sufficient. If the wavelength is a function of more than temperature, a means of measuring the wavelength must be provided to allow for correction of scale factor.
FIG. 2 shows a prior art light source 100 which includes a voltage source 102 for providing an input V.sub.0 to an operational amplifier of a source driver 104. The source driver 104 forces the input V.sub.0 to be the output of the source driver 104 and drives the laser diode 106 having a power output P.sub.0 with a drive current I.sub.F substantially equal to V.sub.0 /R.sub.F. A heater 108 attempts to stabilize the laser diode 106 at a particular temperature. Often, a thermoelectric cooler is used to stabilize the temperature.
In accordance with the discussion above, a change in the laser diode wavelength has the effect of causing a scale factor change which without compensation will produce an inaccurate rotation rate output. Methods for compensating for such wavelength changes of the light source attempt to measure the wavelength change or sense a temperature change only while failing to recognize wavelength dependence on source current. Therefore, there is a need for an alternative source having wavelength compensation to improve scale factor performance. Such a source should be available under all startup temperatures without damaging the light source.